The present invention relates to an antenna device which is useful mainly in a small portable receiver such as a pager, and a radio receiver using such an antenna device, and particularly to a configuration in which stable reception performance can be ensured in various use modes.
In a small portable receiver such as a pager which is to be attached to a human body, a loop antenna which exhibits excellent antenna characteristics in the vicinity of the human body is mounted in a case. Usually, such a loop antenna has a metal loop element which has a rod-like or plate-like shape and which is mounted on a circuit board, and is connected a low-noise amplifier of a receiving circuit via a matching circuit.
A loop antenna reacts to the magnetic field component of an electromagnetic wave rather than the electric field component. On the other hand, a human body can be approximately deemed as a conductor. In the vicinity of a human body, the electric field is weakened because of an electric image, but the magnetic field is conversely strengthened. Therefore, the antenna characteristics of a loop antenna in the case where the antenna is used in the vicinity of a human body are superior to those in the case where the antenna is used in another place.
Recently, it is requested to ensure high reception performance also when a receiver is located in a place separated from a human body, for example, in a handbag or on a table. To comply with this, a technique in which an external antenna is mounted on an external battery case is proposed in, for example, Japanese patent publication (Kokai) 6-140963.
In a conventional small portable receiver having a built-in internal loop antenna, the reception performance depends on the antenna characteristics of the single loop antenna. Consequently, such a receiver has a problem in that the reception performance for a wave reaching from a direction where the antenna directional gain is low, or a wave having a plane of polarization which is different from that of the loop antenna cannot be enhanced.
Moreover, such a loop antenna device has a problem in that, when a small portable receiver is not attached to a human body, the free space gain of the antenna itself cannot be increased.
In a conventional small portable receiver to which an external antenna is connected in order to improve the reception performance in the case where the receiver is separated from a human body, there is a problem in that a large number of terminals must be provided for connection and hence the configuration is complicated.
Furthermore, a conventional small portable receiver having internal and external antennas has a problem in that a loss due to a switching circuit for switching over the internal and external antennas lowers the reception performance.